Environmental Engineering Reference
In-Depth Information
environmental conditions as the result of one or more
' disturbance factors ' . If the initial or ' starting state ' has
persisted for a relatively long time, we can refer to it as
a 'steady state', and it is kept within boundaries by the
system ' s resistance and resilience . In ecological
systems, a steady state is considered to be a dynamic
equilibrium, not truly static or immobile.
Resistance is a characteristic of systems that show
relatively little response to a disturbance factor in terms
of their structural and functional attributes.
Resilience is a characteristic of systems that can be
altered relatively easily by a disturbance factor but then
regain their former structural and functional attributes
in a relatively short time. The length of time a system
requires to return to a former steady state is inversely
related to its resilience; the faster the system returns to
State A of Figure 1.2, the more resilient it is.
The choice of parameters to measure ecosystem sta-
bility is of utmost importance. Seeking to implement
the aforementioned defi nitions of resilience and resis-
tance, Mitchell et al . (2000) combined measurements
of species attributes with environmental variables,
which can represent attributes of either ecosystem
structure or functionality. Their multivariate model-
ling approach, fi rst applied to the conservation man-
agement of lowland heaths in Dorset, United Kingdom,
helps assess why some ecosystems are more resilient
than others.
As mentioned, a disturbed ecosystem is sometimes
described as having crossed over thresholds or even
thresholds of irreversibility , indicating that changes
or switches have occurred that are severe and diffi cult
to reverse without more or less important human
intervention. In Chapter 20 the authors refer to this
concept of 'thresholds' as a tool for determining the
degree of ecosystem resilience and apply it to evaluate
the effects of invading alien species. Once a threshold
is passed, the system is considered disturbed. Similarly,
a degraded ecosystem itself may remain in the dis-
turbed state, that is, the alternative steady state can
also be resilient through internal feedback that con-
strains restoration (Suding et al . 2004 ; cf. Folke
et al . 2004). Very often removing a disturbance factor
will not result in recovery of components of the pre-
existing ecosystem that have been 'lost'. For example,
just rewetting a drained wetland will not be suffi cient
to insure return of the 'original' or pre-existing species
of that ecosystem to that site (Chapter 16). Similarly,
reduction of nutrient loading in turbid, eutrophied
shallow lakes rarely leads to a satisfactory recovery of
a condition of clear water, indicative of a restored lake,
even if the nutrient level is considerably reduced
(Chapter 18). The discrepancy between the route to
recovery or restoration and the initial route to degra-
dation is known as hysteresis . Current knowledge on
alternative stable states, or 'catastrophic shifts', can be
helpful to understand and explain both disturbance
and restoration processes, and to develop early
warning signals for so-called critical transitions - also
known in the popular literature as ' tipping points ' -
both in ecosystems and in human societies (Scheffer
et al . 2009 ).
2.4.3 Ecosystem health and stress,
and landscape integrity
We now consider three useful, but confusing, meta-
phors, often used to indicate the state of an ecosystem
or a landscape as if these systems are a super-organ-
ism, which is of course not the case. Although it seems
only a small step to elaborate the notion of ecosystem
stability towards defi ning terms such as 'ecosystem
health' and 'landscape integrity', these terms may be
associated with an improper interpretation of holism.
Similarly, the term 'ecosystem stress' is sometimes used
metaphorically to describe the state of an ecosystem,
as if the physiological state of an ecosystem could be
compared to that of an individual organism. As scien-
tists, we may have reservations about metaphors and
analogies, but we are obliged to work with useful terms
such as ecosystem health , stress and integrity , as they
can help in communication and consensus building
wherever ecological and socio-economic valuation
systems meet (Aronson et al . unpubl. MS ).
Ecosystem health has been described as 'the state
or condition of an ecosystem in which its dynamic
attributes are expressed within normal ranges of activ-
ity relative to its ecological state of development' (SER
2004). It can be ecologically evaluated in terms of the
state of ecosystem functioning at a given time (Winter-
halder et al . 2004 ), but socio - economic criteria should
also be taken into account (Rapport et al . 1998 ). Rivers,
for example, are not just ecosystems, but can also be
considered as sources of clean water for drinking and
washing, for industrial and agricultural purposes, as
conduits for pollutants, and as places for recreation
and aesthetic pleasure. Ecosystem stress then indi-
cates the state of an unhealthy ecosystem, outside the
optimal environmental range, which can be caused by
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